/Engine/Plugins/Online/OnlineSubsystem/Source/Public/OnlineSubsystemImpl.h
Get the instance name, which is typically "default" or "none" but distinguishes one instance from another in "Play In Editor" mode. Most platforms can't do this because of third party requirements that only allow one login per machine instance
the instance name of this subsystem
virtual [FName](API\Runtime\Core\UObject\FName) GetInstanceName() const